Apparatus and process for reducing coking of heat exchange surfaces

ABSTRACT

A heat exchange surface in reactors and/or heat exchangers of installations for the conversion of hydrocarbons and other organic compounds at high temperatures in the gaseous phase. According to the invention, the metallic surfaces coming into contact with the organic substances are treated at a temperature of 300 to 1000° C. over a period of 0.5 to 12 hours with a mixture of a silicon- and sulfur-containing product and a dry gas flow which is inert with respect to the silicon- and sulfur-containing product. The invention is further directed to a process for producing a catalytically inactivated metallic surface in chemical reactors and/or heat exchangers.

FIELD OF THE INVENTION

The invention is directed to heat exchange surfaces in reactors andtubular heat exchangers in installations for converting hydrocarbons andother organic compounds in relation to the problem of coke formation onthese exchange surfaces.

BACKGROUND OF THE INVENTION

In order to produce ethylene and other lower olefins, hydrocarbons ormixtures of hydrocarbons are thermally cracked, for example inexternally heated reactors formed of metallic materials and the hotcracked products obtained thereby are cooled after leaving the crackingfurnace in heat exchanger apparatuses which are operated externally withwater under pressure serving as coolant.

The cracking furnaces are preferably formed of high-temperature steelscontaining chromium and nickel. The tubular heat exchangers arepreferably formed of low-alloy steels or boiler construction steel. Thisapparatus can also be used to produce other organic products, e.g., asin the production of vinyl chloride by pyrolysis of 1,2-dichloroethane.

The operating efficiency of such apparatus formed of metallic materialsis highly dependent on the extent of carbon-rich deposits forming attheir inner surfaces during operation. Such deposits can not only impedethe desired heat transfer, but can also reduce the free cross section ofthe employed tubes which is important for maintaining throughput. Thisis true of currently used apparatus 25. FIG. 1 shows a typical curve Afor the dependence of the quantity of deposited coke-like products m onthe reaction time t.

After a certain period of operation, the deposits formed on the sides ofthe apparatus coming into contact with the organic compounds reach apermissible coke layer thickness S, as shown in FIG. 1, which causesreductions in output and necessitates the shutdown of operations andcostly cleaning procedures. The coke-like deposits are usually removedby gasification using a mixture of hot steam and air which uncovers themetallic surfaces and ensures the desired heat flow.

In spite of thorough removal of the deposited coke, the newly formingdeposits can again lead to compulsory shutdown and coke removalprocedures already after a relatively short period of operation (e.g.,20 to 60 days). Since the applied oxidative decoking proceduressimultaneously bring about a change in the material surfaces, suchdecoking procedures always involve an increase in the catalytic activityof the material surfaces which promotes unwanted surface coking. Thiscatalytic activity increases with the number of decoking procedures towhich the respective heat exchange surface is subjected and theoperating periods between decoking procedures decline steadily. This isundesirable for technical reasons as well as from an economic viewpointbecause it not only prevents maximum periods of stationary operatingstates, but also reduces the effective use of the installation andresults in increasingly frequent cleaning costs. For these reasons,efforts have been made for years to find solutions for preventing rapidcoking of the inner surfaces of such apparatus. In order to achieve thisobjective, it has been suggested, among other things, to prevent theformation of catalytically active centers or to inhibit such formationon the inner surfaces of tubes of the respective apparatus by developingpassivating oxide coats, as described in U.S. Pat. No. 3,919,073; tocoat the inner walls of the tubes with thin coats of low-alloy ornickel-free steels, as described in German patent publication DE-A 32476 568, to generate supporting layers or diffusion layers of chromium,as described in the publication by Brown, S. M. and Albright, L. F. ACSSymp. Ser. 32 (1976) 296, aluminum, as described in the publication byFrech, K. J., Hopstock, F. H. and Hutchings, D. A. ACS Symp. Ser. 32(1976) 197, or silicon, as described in the publications by Brown, D.E., Clark, J. T. K., Foster, A. J., McCaroll, J. J. and Simms, M. L. ACSSymp. Ser. New York 202 (1982) 23; Bach, G., Zychlinski, W., Zimmermann,G., Kopinke, F. D. and Anders, K. Chem. Techn. Leipzig 42 (1990) 146;Ansari, A. A., Saunders, S. R. J., Bennett, M. J., Tuson, A. T., Ayers,C. F. and Steen, W. M. Materials Science and Engineering 88 (1987) 135;and to add additives in the form of gas or steam of sulfur-containingcompounds, as described in the publication by Boene, K. Oilgas J. 81(1983) 93, phosphorus-containing compounds, as described in thepublication by Gosh, K. K. and Kunzru, D. Ind. Engng. Chem. Res. 27(1988) 559 and in U.S. Pat. Nos. 4,835,332; 4,842,716; and 4,900,426,and nitrogen-containing compounds, as described in the publication byEgiasarov, J. G., Cores, B. Ch. and Potapova, L. L. "Neftechimija."Erdolchem 25 (1985) 627, to the charging product.

As disclosed in U.S. Pat. Nos. 4,835,332; 4,842,716; and 4,900,426 it isknown to reduce the formation of coke-like deposits on the innersurfaces of reactors by adding organic phosphorus compounds. The organicphosphorus compounds (including organic thiophosphorus) can be used assuch or as constituents of special compounds. The addition of organicphosphorus compounds is always linked with the formation of more or lessvolatile phosphines which are not only toxic but can also lead tocatalyst contamination in the downstream processes. The addition oforganic phosphorus compounds is effective only within a limited scope.

Contradictory assertions have been made, such as those disclosed inCzechoslovakian patent publication CS-A 180861 and in the publication byFroment, G. F. Reviews in Chem. Eng. 6(4) (1990) 293, concerning theeffect of sulfur compounds on coking. Nevertheless, sulfur compounds arefrequently used in industrial practice hydrocarbon fractions (naphtha,kerosine, gas oil, etc.), the addition of sulfur compounds has hardlyany discernable effect on coking. They contain ad hoc sulfur compoundsas mixture components. However, a more or less pronounced formation ofcoke-like deposits is observed during the pyrolysis of such hydrocarbonfractions.

In addition, although the application of oxidic protective coatings, asis suggested, in European patent publication EP-A 0 110 486, would leadto improvements, it cannot be considered a satisfactory solution.

A further improvement is provided by a coating based on silicon oilwhich is subsequently thermally decomposed under strictly specifiedconditions to produce a protective layer, as described in thepublication Chem. Tech. Leipzig 42 (1990) 146. This process, like theproduction of laser-induced SiO₂ surface layers, is relatively costlyand the generated SiO₂ layers are not stable during changes in thetemperature of the outer tube wall in the range of 750 to 1100° C. Thisalso applies to any passivated layers obtained by the silica coatingwhich is described by British Petroleum Co. Ltd. in the publication ACSSymp. Ser. New York, 202 (1982) 23-43 in comparison with the publicationChem Techn. Leipzig 42 (1990) 146 ff.

Finally, reference is made to the attempted use of tubes of steel alloyswhose inner surface is coated by thin coats of low-alloy or nickel-freesteels described in German patent publication DE-A 3 247 568. It hasbeen shown that the results of such plating do not justify the effort.

With the exception of the reduction of coke formation through theaddition of phosphorus- and/or sulfur-containing additives to thepyrolysis charging products, all of the proposed solutions describedabove can only be practically carried out in new installations or in newtubing, but not in installations which have already been in use.

Therefore, the object of the present invention is to propose newimproved heat exchange surfaces and to provide a process for reducingcoking by which the respective apparatus (outfitting) of an installationwhich has already been completely installed can be subjected to suchtreatment before being put into operation and also after every decokingprocedure.

SUMMARY OF THE INVENTION

According to the invention, the heat exchange surface in reactors and/orheat exchangers of installations for converting hydrocarbons and otherorganic compounds at high temperatures in the gaseous phase ischaracterized in that the metallic surfaces coming into contact with theorganic substances are treated at a temperature of 300 to 1000° C. overa period of 0.5 to 12 hours with a mixture of a silicon- andsulfur-containing product and a dry gas flow which is inert with respectto the silicon- and sulfur-containing product.

For this purpose, the silicon- and sulfur-containing product is selectedfrom (1) one or more silicon- and sulfur-containing volatile compounds,(2) a mixture of silicon-containing volatile compounds and a mixture ofsulfur-containing volatile compounds, and (3) a mixture of silicon- andsulfur-containing volatile compounds and volatile silicon-containingand/or volatile sulfur-containing compounds, wherein the atomic ratio ofsilicon to sulfur in (1), (2) or (3) is 5:1 to 1:1. Particularlyadvantageous compounds are trimethylsilyl mercaptan, dimethyl sulfide,dimethyl disulfide, and bis(trimethylsilyl) sulfide and mixturesthereof.

If the heat exchange surface which is treated according to the inventionis the metallic inner surface of the tubes of a tubular reactor, thetreatment temperature is 800 to 1000° C. If the heat exchange surfacewhich is treated according to the invention is the metallic innersurface of the tubes of a heat exchanger downstream of the tubularreactor, the treatment temperature is 300 to 750° C. However, in thelatter case a higher temperature can also be employed locally. Thus, thetemperature at the baffle plate at the input of the heat exchanger canalso exceed 800° C. in certain cases, e.g., 875° C. Normally, however,the temperature remains within the range indicated above.

As was already stated, the treatment period is generally 0.5 to 12hours. The effect of a treatment period of less than 0.5 hours is notsufficient to show a long-lasting effect. Periods in excess of 12 hoursare possible, but are generally uneconomical.

The invention is based on the surprising insight that the verysubstantial increase in coking which is always observed when initiallyputting into operation cracking furnaces whose reactor tubes are new orwhose inner surfaces are freed of carbon-rich products which havealready been deposited can be effectively reduced in that the innersurfaces of the tubes coming into contact with the cracked productsafter being put into operation are subjected to a suitablehigh-temperature treatment with silicon- and sulfur-containing volatilecompounds before the cracking furnace is put into operation for thefirst time and/or after every time the crack furnace is put intooperation thereafter subsequent to steam/air decoking. This is advisablyeffected in such a way that a mixture of silicon- and sulfur-containingcompounds and an inert dry carrier gas which receives the compounds uponwhich the invention is based is sent through the tubes of a crackingfurnace and of the tubular heat exchanger connected thereto in acomposition such that the catalytically active centers which are presenta priori on the inner surfaces of the tubes and which are responsiblefor the catalytic coke formation are converted by chemical reactionsinto catalytically passive surface compounds and an enrichment of theelements contained in the compounds according to the invention, namelysilicon and sulfur, takes place in the form of reactive species in thesurface of the metallic materials. When the catalytically active centerson the inner surface of the tubes are converted accompanied by theformation of catalytically inactive surface compounds and the silicon-and sulfur-containing species have penetrated into the material surfaceto a sufficient extent, the cracking furnace, including the tubular heatexchanger, can be put into operation again. Since the coatings on theinner surface of the tubes are enriched, especially in silicon, and thecatalytically active centers are inactivated by the growth of thermallystable and catalytically inactive silicon-sulfur species, a recurrenceof coking will take place only after a long delay and at a very lowlevel, as represented by curve B in FIG. 1. As a result of thiscomparatively simple additional treatment prior to putting a completelyassembled cracking furnace into operation for the first time or afterthe cracking furnace has been subjected to a conventional cleaning bydecoking with a steam/air mixture, the present invention makes itpossible to considerably prolong the operating times of crackingfurnaces. Significantly, the cracking furnaces and tubular heatexchangers themselves need not undergo any structural modification andthe process is also applicable to installations which are already inoperation. There is no need for costly coating of prefabricated tubeswhich must be welded during assembly so that the protective coatings arepartially destroyed and the desired effect is partially cancelled.Furthermore, the application of closed cover layers which can impede thetransfer of heat is avoided.

It has proven advantageous to convey a mixture of an inert, dry carriergas, such as the head product from the demethanizer of the cracked gasdecomposition system or nitrogen, and the compounds according to theinvention through the furnace system at the conventional operatingtemperature for a cracking furnace, i.e., at tube wall temperaturesabove 800° C., and at the usual operating temperature for a tubular heatexchanger (TLE), i.e., at roughly 400 to 550° C., wherein the molarratio of the silicon- and sulfur-containing compounds to the carrier gasis between 0.0005 and 0.03 and a treatment period ranges between 30minutes and 12 hours depending on the concentration of the silicon- andsulfur-containing compounds. In addition to compounds containing siliconand sulfur simultaneously, mixtures of silicon-containing andsulfur-containing compounds can also be used. The atomic ratio ofsilicon to sulfur can range between 5:1 and 1:1, preferably between 1:1and 2:1. The pressure of the mixture sent through the system cancorrespond to the usual pressures in a cracking furnace system, e.g.,0.5 to 20 bar, preferably in a range of 1 to 2 bar. A carrier gas otherthan the inert gas for the system can also be used.

The invention will be explained more fully in the following withreference to a number of comparison examples and embodiment examplesaccording to the invention. FIGS. 2 to 10 illustrate the dependency ofthe coking rates in preactivated test pieces of chromium-nickel steel onthe test period during the pyrolysis of n-heptane, in some cases afterthermal pretreatment according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the dependency of the amount of deposited coke-likeproducts on the reaction time t in an apparatus according to the priorart;

FIG. 2 shows an example of the dependency of the coke-forming rate in atest piece of chromium-nickel steel X 8 CrNiTi 18 10 which has beenpreactivated (E=decoking with air) but not pretreated, according to theinvention, on the test period during pyrolysis of pure n-heptane (T_(R)=715° C., τ=1 s, N₂ as diluent);

FIG. 3 shows the influence of 85 ppm dimethyl disulfide (DMDS), as anaddition to n-heptane, on the rate of coke formation in a test piece ofX 8 CrNiTi 18 10 which has been preactivated but not pretreated,according to the invention, in relation to the test period duringpyrolysis of n-heptane (T_(R) =715° C., τ=1 s, N₂ as diluent);

FIG. 4 shows the influence of 1000 ppm triphenylphosphine oxide (TPPO)instead of dimethyl disulfide as an addition to n-heptane on the rate ofcoke formation in a test piece of X 8 CrNiTi 18 10 which has beenpreactivated but not pretreated, according to the invention, in relationto the test period during pyrolysis of n-heptane (T_(R) =715° C., τ=1 s,N₂ as diluent);

FIG. 5 shows the dependency of the rate of coke formation on apreactivated test piece of X 8 CrNiTi 18 10 which has already beendecoked multiple times and thermally pretreated at 880° C. according tothe invention with trimethylsilylmethyl mercaptan in relation to thetest period during pyrolysis of n-heptane and with repeated interruptionof the pyrolysis reaction for the purpose of burning off deposited cokeby means of air (T_(R) =715° C., τ=1 s, N₂ and steam, respectively, asdiluent);

FIG. 6 shows the dependency of the coking rate on the test period in atest piece of unused preactivated Incoloy 800 which has been pretreatedaccording to the invention in relation to the test period duringpyrolysis of n-heptane and with repeated interruption of the pyrolysisreaction for the purpose of burning off deposited coke by means of air(T_(R) =715° C., τ=0.6 s, steam as diluent);

FIG. 7 shows the influence of the carrier gas used for the thermalpretreatment of the test piece of X 8 CrNiTi 18 10 on the coking rateduring the pyrolysis of n-heptane (T_(R) =715° C., τ=1 s, N₂ asdiluent);

FIG. 8 illustrates the temperature influence in the pretreatment,according to the invention, of the test piece of X8 CrNiTi 18 10 on thedependency of the coking rate on the test period during the pyrolysis ofn-heptane (T_(R) =715° C., τ=1 s, N₂ as diluent);

FIG. 9 illustrates the influence of the pretreatment time on thedependency of the coking rate on the test period during the pyrolysis ofn-heptane (T_(R) =715° C., τ=1 s, N₂ as diluent);

FIG. 10 shows the dependency of the coking rate on different pretreatedtest pieces of X 8 CrNiTi 18 10 on the test period during the pyrolysisof n-heptane (T_(R) =715° C., τ=1 s, N₂ as diluent).

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS EXAMPLE 1Comparison Example

The deposition rates of solid, coke-like deposits on metallic materialsduring the pyrolysis of hydrocarbons can be measured in specialvertically arranged, electrically heatable laboratory reactors when thecorresponding material test pieces are suspended within these reactorson a thin platinum or quartz wire and are connected with a thermalscale, in comparison with that described in the publication by Kopinke,D., Bach, G. and Zimmermann, G. J. Anal. Appl. Pyrolysis 27 (1993) 45.

In a pyrolysis apparatus of this kind made from silica glass (d_(i) =20mm; V_(R) =13 ml) to which is connected a separately heated tube segmentof silica glass of identical diameter in which gas chamber temperaturescorresponding to those used in industrial tubular heat exchangers forcooling pyrolysis gases can be simulated, n-heptane as model hydrocarbonwas pyrolyzed at temperatures between 715 and 800° C. under conditionsleading to an ethylene-to-propylene mass ratio in the pyrolysis gasbetween 2.0 and 2.7. When pyrolysis is carried out in nitrogen asdiluent (n_(heptane) :n_(N).spsb.2 =0.5) and in the presence of materialtest pieces on which coke has been deposited repeatedly in order tobring about increased catalytic coke formation by pyrolysis and in whichthe coke was subsequently burned off, absolute coking rates r can bemeasured subsequently, these coking rates preferably ranging betweenr=50 and 300 μg/cm² ·min. The level of the measured coking rates is anintegral measurement value which, at a defined cracking intensity andunder defined cracking conditions, is characteristic of the respectivemeasured test piece, but also depends to a great extent on the number ofcoking/decoking cycles undergone by the respective test piece. A typicalexample for the dependency of the coking rate in a test piece ofchromium-nickel steel X 8 CrNiTi 18 10 on the reaction time duringpyrolysis of n-heptane at 780° C. is shown in FIG. 2 for five successivecoking/decoking cycles.

EXAMPLE 2 Comparison Example

In the same apparatus and under external conditions analogous to thosedescribed in Example 1, the curve of the coking rate was firstdetermined on a preactivated test piece of X 8 CrNiTi 18 10 during thepyrolysis of n-heptane at 715° C. over a test period of 60 minutes. Then-heptane, as pyrolysis charging product, was then substituted by an-heptane charge containing 85 ppm dimethyl disulfide, a compound whichis known and used industrially as a coking inhibitor.

FIG. 3 illustrates the curve of the coking rates measured on theemployed test piece as a function of the test period. The aforementionedcharging product was changed repeatedly. The measured differences in thecoking rates confirm the inhibiting effect of dimethyl disulfide on cokeformation on metallic material surfaces.

EXAMPLE 3 Comparison Example

In the same apparatus as that described in Example 1 and under theconditions described in the example, the effect of a knownphosphorus-containing inhibitor, as disclosed in U.S. Pat. No.4,900,426, on the coking rate at 715° C. was plotted instead of thedimethyl disulfide. The results of the investigations are compiled inFIG. 4. It will be seen that an addition of 1000 ppm triphenylphosphineoxide (the P content is standardized to the S content of the compoundused in Example 2) to the n-heptane does not have a discernable effecton its coke-forming tendency under the applied pyrolysis conditions.

EXAMPLE 4 Embodiment Example According to the Invention

In the same apparatus as that described in Example 1, a repeatedlypreactivated test piece of X 8 CrNiTi 18 10 was treated for a period of60 minutes with a 3 l/h flow of gas (volume rate V=25 ml/ml·min) of0.005 moles trimethylsilylmethyl mercaptan in 3 liters of a dryequimolar mixture of hydrogen and methane at 880° C. The reactor wasflushed for 5 minutes with nitrogen at 715° C. Subsequently, n-heptanewas pyrolyzed in the presence of nitrogen (n_(heptane) :n_(N).spsb.2=0.5) at 715° C., as was described in Example 1, and the coking rate onthe pretreated test piece was determined as a function of the reactiontime (FIG. 5). The coking rate of r=4 μg/cm² ·min remained virtuallyconstant over a test period of more than 18 hours. By arbitraryinterruption of the test, the surface of the test piece was cleanedafter 8, 12, and 15 hours by means of burning off the coke with air.There was no impairment of the surface passivity. After 18 test hours,the nitrogen used as diluent was replaced by steam and the test wascontinued for an additional 24 hours. The coking rate dropped to valuesof around 3 μm/cm² ·min and remained virtually constant over theaforementioned test period.

EXAMPLE 5 Embodiment Example According to the Invention

In the same apparatus as that described in Example 1, a test piece ofunused Incoloy 800, as mentioned in Example 4, was pretreated under theconditions indicated in Example 4 and the coking rate during pyrolysisof n-heptane at 750° C. was subsequently plotted. The pyrolysis wascarried out in the presence of steam instead of nitrogen as diluent. InFIG. 6, the measured coking rates were plotted relative to the testperiods. The pyrolysis was interrupted repeatedly and the test piece wasdecoked with air. The results show that the coking rate has low valuesof around 2.5 μm/cm² ·min over the entire testing period.

EXAMPLE 6 Embodiment Example According to the Invention

In the same apparatus as that described in Example 1 and under theconditions described in Example 4, the influence of the carrier gas usedfor pretreatment on the coking rate during pyrolysis of n-heptane wasinvestigated. Hydrogen, methane, nitrogen and steam were used instead ofa 1:1 mixture of hydrogen and methane. The variation in the carrier gasused for pretreatment shows that steam is not suitable for long-lastingsuppression of coking on materials pretreated with trimethylsilylmethylmercaptan. After comparable low initial values (r=1.7 μm/cm² ·min) weremeasured, the coking rate increased continuously and reached a value ofr=25 μm/cm² ·min again after a test period of only 120 minutes.

FIG. 7 shows the coking rates measured after the correspondingpretreatments during pyrolysis of n-heptane at the surface of the testpiece as a function of the test period.

EXAMPLE 7 Embodiment Example According to the Invention

In the apparatus described in Example 1, preactivated test pieces of X 8CrNiTi 18 10 were treated at four different temperatures over a timeperiod of 60 minutes in each instance with a 3 l/h equimolar gas flow ofhydrogen and methane to which 0.005 moles of trimethylsilylmethylmercaptan was added. After this treatment and after flushing the reactorwith nitrogen, the coking rates were measured at the test pieces duringpyrolysis of n-heptane in the presence of nitrogen at 715° C.(n_(heptne) :n_(N).spsb.2 =0.5).

In FIG. 8 the coking rates measured at the test pieces treated withtrimethylsilylmethyl mercaptan at four different temperatures are shownas a function of the reaction time. It will be seen that the treatmentof the material surfaces according to the invention before the pyrolysisof hydrocarbons is dependent on the pretreatment temperature. Atpretreatment temperatures of more than 880° C., the coking is suppressedfor lengthy periods.

EXAMPLE 8 Embodiment Example According to the Invention

Preactivated test pieces of X 8 CrNiTi 18 10 were pretreated at 900° C.over different lengths of time with an equimolar mixture of hydrogen andmethane containing trimethylsilylmethyl mercaptan in the same apparatusas that described in Example 1 and under conditions analogous to thosedescribed in Example 7. The coking rates which were subsequentlymeasured at these test pieces during the pyrolysis of n-heptane innitrogen at 715° C. as a function of the test period are shown for fourtest pieces in FIG. 9.

The variation of the pretreatment period shows that the coke formationcan be suppressed in an equally effective manner in pretreatment periodsgreater than 1 h over lengthy test periods.

EXAMPLE 9 Embodiment Example According to the Invention

In the same apparatus as that described in Example 1 and under the sameconditions as those indicated in Example 4, the influence of the typeand composition of the silicon- and sulfur-containing compounds on thecoking rate during pretreatment of a preactivated test piece by means ofa carrier gas comprising 50 mol-% hydrogen and 50 mol-% methane wasinvestigated during pyrolysis of n-heptane in nitrogen as diluent.

The test pieces which were obtained at a pretreatment temperature of880° C., a pretreatment period of 60 minutes, and with a proportion of0.005 moles of the silicon- and sulfur-containing compound or of the sumof silicon- and sulfur-containing compounds in a 3 l/h equimolarhydrogen-methane mixture were subjected one after the other to thereactive gas phases occurring during pyrolysis and the coking rates atthis test pieces were measured as a function of the reaction time.

Table 1 shows the coking rates which were obtained at the test piecespretreated with different silicon- and sulfur-containing compounds as afunction of the test period.

It will be seen that the object of the pretreatment according to theinvention is not limited to the use of compounds simultaneouslycontaining silicon and sulfur. Rather, this object is also met whencompounds containing silicon or sulfur are applied in a mixture. In sodoing, the pretreatment according to the invention is ensured over awide range of atomic ratios of silicon to sulfur. A particularlyadvantageous ratio is Si:S=2:1 to 1:1.

EXAMPLE 10 Embodiment Example According to the Invention

In the same apparatus as that described in Example 1 and underconditions analogous to those indicated in Example 4, the influence ofthe content of trimethylsilylmethyl mercaptan in the equimolarhydrogen-methane mixture used for pretreatment on the coking rate intest pieces of X 8 CrNiTi 18 10 was determined. Differing amounts oftrimethylsilylmethyl mercaptan (0.002, 0.005, 0.01, and 0.02 moles) wereadded to the hydrogen-methane mixture (3 l/h) used for the pretreatmentand the pretreatment was carried out in each instance with 3 l of theconditioned carrier gas indicated above over a period of 60 minutes at880° C.

The coking rates measured at the test pieces which were pretreateddepending on the trimethylsilylmethyl mercaptan content in thehydrogen-methane mixture during the pyrolysis of n-heptane in thenitrogen flow at 715° C. are shown in Table 2.

The results showed no substantial dependency between the measured cokingrates and the trimethylsilylmethyl mercaptan content in thehydrogen-methane mixture used for the pretreatment.

EXAMPLE 11 Comparisons and Invention

In a laboratory pyrolysis apparatus according to Example 1, four testpieces of X 8 CrNiTi 18 10 were treated in each instance over a timeperiod of 60 minutes at 880° C. with a 3 l flow of gas containinghydrogen and methane in equimolar amounts, to which were added 0.005mole tetramethylsilane (test piece PK 1) or dimethyl sulfide (test piecePK 2) or a 1:1 mixture of tetramethylsilane and dimethyl sulfide (testpiece PK 3) or trimethylsilylmethyl mercaptan (test piece PK 4).Accordingly, only test pieces PK 3 and PK 4 were treated according tothe invention. All four test pieces were subsequently subjected, oneafter the other, to the reactive gas phase occurring in the pyrolysis ofn-heptane in the nitrogen flow at 715° C. (dwell period 1 s) and thecoking rates on these test pieces were measured as a function of theduration of the pyrolysis tests. The results are shown in the form of agraph in FIG. 10. A comparison shows that the low coking rates typicalfor all test pieces were maintained over long test periods only in testpieces 3 and 4 which were pretreated according to the invention. It mustbe concluded from the determined data that the pretreatment according tothe invention enables a significantly prolonged operating time comparedto an operation without pretreatment or with a compound containing onlysilicon or sulfur.

                  TABLE 1    ______________________________________    Influence of the ratio of silicon to sulfur in the inert gas (total    content    of Si--S additive: 0.005 moles) used for the pretreatment of    preactivated test pieces of X 8 CrNiTi 18 10 (880° C., 60 min) on    the    coking rate r during the pyrolysis of n-heptane in the nitrogen flow            a)     b)    c)      d)  e)    f)  g)    ______________________________________    atornicratio              1:1      1:1   2:1   2:1 3:1   4:1 5:1    Si:S    test period  min!              r μ· cm.sup.-2 · min.sup.-1 !     10       3.0      2.9   2.8   3.0 3.5   3.8 4.8     30       3.1      3.2   3.0   3.0 4.0   4.2 5.0     50       3.0      3.0   2.9   2.8 4.0   4.4 5.5     70       3.1      3.0   3.0   3.1 4.1   4.5 5.2     90       3.2      3.3   3.1   3.2 4.2   4.7 5.8    100       3.2      3.2   3.0   3.3 4.3   4.6 5.6    ______________________________________     Si, S compounds used for pretreatment:     a) trimethylsiiylmethyl mercaptan     b) 1: 1 mixture of tetramethylsilane and dimethyl sulfide     c) bis(trimethylsilyl) sulfide     d) 2: 1 mixture of tetramethylsilane and dimethyl sulfide     e) 3: 1 mixture of tetramethylsilane and dimethyl sulfide     f) 4: 1 mixture of tetramethylsilane and dimethyl sulfide     g) 5: 1 mixture of tetramethylsilane and dimethyl sulfide

                  TABLE 2    ______________________________________    Dependency of the coking rate r on the trimethylsilylmethyl mercaptan    content in the inert gas of the thermal pretreatment of test pieces of X    CrNiTi 18 10 during the pyrolysis of n-heptane in the nitrogen flow    Content of trimethylsilyl-    methyl mercaptan in the    inert gas     mol!            0.002  0.005    0.01 0.02    test period  min!                     r  μg · cm.sup.-2 · min.sup.-1    ______________________________________                                        !    10               3.5    3.0      2.9  2.9    30               3.5    3.1      2.9  2.8    50               3.4    3.0      3.0  2.9    70               3.6    3.1      3.0  3.0    90               3.8    3.2      2.9  2.8    120              3.7    3.2      3.1  2.9    ______________________________________

We claim:
 1. A process for reducing coking of a heat exchanger metallicsurface formed after a period of producing thermally cracked productsfrom hydrocarbons in organic compounds, comprising:contacting themetallic surface of the heat exchanger with a mixture of a silicon- andsulfur-containing product and a dry inert gas flow at a temperature of300 to 1000° C. over a period of approximately 0.5 to 12 hours at leastone of before and after cracking takes place.
 2. The process of claim 1,said contacting step further comprises contacting the metallic surfaceat least one of before initial startup of operation and after cleaningof the metallic surface.
 3. The process of claim 1, wherein the silicon-and sulfur-containing product is one of: a) at least one silicon- andsulfur-containing volatile compound; b), a mixture of silicon-containingvolatile compounds and sulfur-containing volatile compounds, and c) amixture of silicon- and sulfur-containing volatile compounds and atleast one of volatile silicon-containing and volatile sulfur-containingcompounds, wherein an atomic ratio of silicon to sulfur in the silicon-and sulfur-containing product is between 5:1 and 1:1.
 4. The process ofclaim 3, wherein a molar ratio of the silicon- and sulfur-containingcompound or the mixture of silicon- and sulfur-containing compounds tothe inert gas is between 0.001 and 0.01.
 5. The process of claim 4,wherein the molar ratio is between 0.001 and 0.004.
 6. The process ofclaim 1, wherein the period is between approximately 0.5 to 8 hours. 7.The process of claim 1, wherein the period is between approximately 1 to6 hours.
 8. The process of claim 1, wherein the metallic surfacecomprises an inner tube surface of a tubular reactor subjected to cokingand is contacted with the product at the temperature of 700 to 1000° C.9. The process of claim 1, wherein the metallic surface comprises asurface of a heat exchanger subjected to coking and is contacted at thetemperature of 300 to 750° C.
 10. The process of claim 1, wherein theinert gas exits a reactor and is fed to the heat exchanger at atemperature above 500° C.
 11. The process of claim 1, wherein the inertgas is one of nitrogen, hydrogen, and methane- and hydrogen-containinggases.
 12. The process of claim 1, wherein the inert gas is methane- andhydrogen-containing residual gases from a column gas separation.
 13. Theprocess in accordance with claim 1, wherein the silicon- andsulfur-containing product consists of an organosilicon and anorganosulfur containing product.
 14. The process in accordance withclaim 13, wherein the silicon- and sulfur-containing product includes ofcarbon and hydrogen atoms.
 15. The process in accordance with claim 13,wherein the silicon- and sulfur-containing product comprises a siliconatom bonded adjacent to a sulfur atom.
 16. A heat exchanger surfaceincluding a metallic surface treated in accordance with claim
 1. 17. Theheat exchanger of claim 16, wherein the silicon- and sulfur-containingproduct is one of: a) at least one silicon- and sulfur-containingvolatile compounds; b) a mixture of silicon-containing volatilecompounds and sulfur-containing volatile compounds; and c) a mixture ofsilicon- and sulfur-containing volatile compounds and at least one ofvolatile silicon-containing and volatile sulfur-containing compounds,wherein an atomic ratio of silicon to sulfur in the silicon- andsulfur-containing product is between 5:1 and 1:1.
 18. The heat exchangerof claim 16, wherein the metallic surface comprises an inner tubesurface of a tube reactor and is contacted with the product at thetemperature of 700 to 1000° C.
 19. The heat exchanger of claim 16,wherein the metallic surface comprises an inner tube surface of a tubereactor and is contacted with the product at the temperature of 800 to1000° C.
 20. The heat exchange surface of claim 16, wherein the metallicsurface comprises a surface of a heat exchanger and is contacted withthe product at the temperature of 300 to 750° C.